Exploration of nonlinear optical properties of 4-methyl-4H-1,2,4-triazol-3-yl)thio)-N-phenylpropanamide based derivatives: experimental and DFT approach

Triazoles, nitrogen-containing heterocycles, have gained attention for their applications in medicinal chemistry, drug discovery, agrochemicals, and material sciences. In the current study, we synthesized novel derivatives of N-substituted 2-((5-(3-bromophenyl)-4-methyl-4H-1,2,4-triazol-3-yl)thio)-N-phenylpropanamide and conducted a comprehensive investigation using density functional theory (DFT). These novel structural hybrids of 1,2,4-triazole were synthesized through the multi-step chemical modifications of 3-bromobenzoic acid (1). Initially, compound 1 was converted into its methyl-3-bromobenzoate (2) which was then transformed into 3-bromobenzohydrazide (3). The final step involved the cyclization of compound 3, producing its 1,2,4-triazole derivative (4). This intermediate was then coupled with different electrophiles, resulting in the formation of the final derivatives (7a–7c). Additionally, the characterization of these triazole-based compounds (7a, 7b, and 7c) were carried out using techniques such as IR, HNMR, and UV–visible spectroscopy to understand their structural and spectroscopic properties. The DFT study utilized M06/6-311G(d,p) functional to investigate geometrical parameters, HOMO–LUMO energies, natural bond orbital analyses, transition density matrix (TDM), density of states, and nonlinear optical (NLO) properties. The FMO analysis revealed that compound 7c exhibited the lowest band gap value (4.618 eV). Notably, compound 7c exhibited significant linear polarizability (4.195 >  × 10–23) and first and second hyperpolarizabilities (6.317 >  × 10–30, 4.314 × 10–35), signifying its potential for nonlinear optical applications. These NLO characteristics imply that each of our compounds, especially 7c, plays a crucial part in fabricating materials showing promising NLO properties for optoelectronic applications.


General
Standard chemicals from Sigma Aldrich, Acros Chemicals, Macklin, and TCI were used without any additional purification.Silica gel 60 254F plates (Merck Germany) was used to perform TLC for reaction progress.UV-visible spectra were measured and analyzed with high accuracy and precision by the Hitachi UV-Visible Double Beam Spectrophotometer.A Stuart melting point apparatus was used to determine the melting point of the samples.

Results and discussion
The spectroscopic examination identified synthesis of the compounds, as illustrated in Figs.S1-S21.Following the determination of their structural conformation, a DFT investigation was conducted on the newly synthesized molecules featuring 1,2,4-triazole functionalities.The specifics of this exploration are discussed below.

Frontier molecular orbitals (FMO)
To explore the optical as well as electronic properties of compounds, FMO analysis is a remarkable tool to evaluate the probability of intramolecular charge transfer (ICT) 50,51 .HOMO demonstrates the ability to donate an electron and is regarded as valence band, while LUMO reflects the capacity to accept an electron and is regarded as conduction band.FMOs are the substantial source of transition energies that occur when an electron excites from the HOMO to the LUMO.The energy difference (∆E = E LUMO -E HOMO ) of compounds is directly associated with their kinetic stability and chemical reactivity.A reduced energy difference (∆E) value results in increased molecular polarizability, leading to an enhanced nonlinear optical (NLO) response 52 .E LUMO , E HOMO, and ∆E values for A-C were calculated, and the outcomes are in Table 1.
The findings from Table 1 illustrates that by varying different groups in fragment 3, band gap of synthesized compounds vary greatly.Compounds with large ∆E need more HOMO to LUMO energy of transition than those with less energy difference hence making them preferable for NLO devices [53][54][55] .Therefore, compounds with lower band gaps are compelling in the opto-electronic domain.In these synthesized compounds, fragments 1 and 2 are kept unaltered, while fragment 3 is modified, which results in different ΔE values.The HOMO energies of synthesized molecules 7a, 7b, and 7c are determined as − 7.128, − 6.973, and − 7.144 eV, respectively, whereas the LUMO energies are calculated as − 1.491, − 1.358, and − 2.526 eV, respectively.Consequently, the DFT-based computed HOMO-LUMO energy gaps of synthesized molecules are 5.637, 5.615, and 4.618 eV, respectively.
Among the synthesized compounds, the maximum ∆E value (5.637 eV) is found in A, which has a chloro group at the para position, and CF 3 group at the meta position.Due to negative inductive effect (−I) the chloro group only slightly withdraws electrons, and it also releases electrons via resonance.Here, the resonance effect dominates over the inductive effect and thus reduces the charge transference.Moreover, the CF 3 group is an electron withdrawing group due to the presence of three fluoro groups on the same carbon atom.Subsequently www.nature.com/scientificreports/B with two chloro groups in its structure showed reduced band gap.This chloro group at the ortho position has a stronger electron-withdrawing effect, it would increase the charge transference in the molecule and potentially reduce the band gap.Furthermore, the compound C, with a meta-positioned NO2 group, exhibits the lowest band gap value, ascribed to the strong electron-withdrawing nature of the nitrogen atom, resulting in improved charge transfer and a pronounced push-pull mechanism.Overall, the energy gap is observed in the following decreasing order: A > B > C. Additionally, the outcomes for HOMO-1, LUMO + 1, HOMO-2, and LUMO + 2 energies, as listed in Table S1, provides further insights into the electronic structure and exciton dynamics of these compounds.Hence, among the three synthesized molecules (A, B, C) a smaller ∆E value is observed in C. The orbital diagrams of HOMO and LUMO are presented in Fig. 3.For HOMO, the charge density of synthesized compounds resides mainly on fragments 1 and 2, while LUMO charge density is hugely located on fragment 1 and partly on fragment 2.These orbital diagrams are useful for analysing effective charge transfer in the synthesized compounds.Therefore, high charge transition makes the synthesized compounds efficient materials for advanced NLO devices.

Global reactivity parameters (GRPs)
Band gap is the most significant factor for the estimation of the global reactivity parameters (GRPs), such as electron affinity (EA) 56 , ionization potential (IP) 57 , global hardness (η) 58 , global softness (σ) 59 , chemical potential (μ) 60 , global electrophilicity index (ω) 61 , and electronegativity (X) 62 .It is an effective approach to predict global trends in chemical reactivity using these characteristics.The EA and IP amplitudes of chemical compounds are used for identifying their ability to capture and donate electrons 63  www.nature.com/scientificreports/ in chromophores relies on these parameters, where higher values of chemical potential (μ) and hardness (η) indicate increased kinetic stability.Additionally, these variables are directly linked to ∆E values and exhibit an inverse relationship with global softness (σ).Therefore, molecules having smaller ∆E are more reactive, stable, and soft with high polarization and serve as a finer competitor in providing the best NLO response 64 .Table 2 shows the outcomes for the computed GRPs of the examined chromophores.By using Koopmans′ theorem (Eqs.1-7), the chemical hardness (η), chemical potential (μ), electronegativity (X), global softness (σ), and electrophilicity index (ω) are calculated 65,66 .Electronegativity (X) and chemical potential (μ) are calculated using Koopmans's theory as: The global softness (σ) and chemical hardness (η) can be calculated through Eqs. ( 5) and ( 6).
The electrophilicity index (ω) is reported by Parr et al. 43,67 .as: From Table 2, compound C exhibits the highest ionization potential (IP) value among the synthesized chromophores (7.144 eV), suggesting efficient charge distribution among its fragments.In contrast, compound B exhibits the lowest ionization potential (IP) value of 6.973 eV, while compound C has the highest value of 7.144 eV.The ionization potential values follow the order B < A < C. Additionally, compound C demonstrates the highest electron affinity (EA) value of 2.526 eV, while compound B has the lowest value of 1.358 eV.It is noteworthy that the ionization potential values are generally higher than the electron affinity values for compounds A, B, and C. According to the literature, higher IP values indicate more chemical inertness and stability 68 .Chemical potential (μ) describes the molecular stability 69 by relating to molecular electronegativity.The negative μ value indicate that the compound accepts electrons readily 70 .Similarly, the negative chemical potential values shown by the synthesized compounds indicate their stable nature.
The order of global hardness (η) follows the trend C < B < A, correlating with the increasing energy gap (E gap ).This indicates that harder molecules with higher ∆E values exhibit greater stability and lower reactivity.Global softness (σ) is also linked to chemical potential and plays a role in understanding reactivity and stability.The increasing order of σ is given as A < B < C.This ascending order is opposite to the increasing energy gap order, representing A (0.177 eV) with the least reactivity; however, C (0.216 eV −1 ) is highly reactive showing highest softness value among the synthesized compounds.Overall, E gap order and global reactivity descriptors have a very good association.Low-lying E gap is admitted to showing high molecular nonlinear behavior.The remarkable nonlinear optical (NLO) responses observed in the tested systems strongly indicate their potential for significant applications in optoelectronics.

Density of states (DOS) analysis
The DOS further supports the findings of the FMO analysis by confirming electronic delocalization in the HOMO and LUMO orbitals 71 .Determining the distribution of electron charge on molecular orbitals by computing the DOS percentages for HOMOs and LUMOs offers additional support for the idea that the electron charge patterns are influenced by the differences in fragment 3. To comprehend the DOS, we segmented our chromophores into three components, namely, fragments 1, 2, and 3.The illustration in Fig. 4 depicts these fragments using red, green, and blue lines, correspondingly.Along the x-axis of DOS graphs, negative values denote the valence band (1)  72 .Figure 4 illustrates that the HOMO electron density of compound A is primarily located on fragment 3, while the LUMO charge density is predominantly distributed on fragment 3 and partially on fragment 2. In case of B, the HOMO charge density is mostly found on fragment 2, and fragment 3 having a small contribution, while the LUMOs are dispersed across fragment 2. For compound C, HOMOs are evenly distributed throughout fragments 2 and 3, but the LUMOs of the same compounds are located only on fragment 2. Table S9 demonstrates that for compounds A-C, fragment 1 contributes 46.8, 36.1, and 49.7% to HOMO and 53.5, 59.9, and 0.0% to LUMO, respectively, while fragment 2 contributes 51.5, 60.8, and 49.6% to HOMO and 40.8, 39.4, and 0.4% to LUMO, accordingly.Similarly, fragment 3 contributes 1.7, 3.1, and 0.7% to HOMO and 5.7, 0.7, and 99.6% to LUMO, respectively.The maximum HOMO charge density for compound A appears on fragment 2 at − 6.5 eV, while the highest LUMO charge density is found on fragment 3 at 1.5 eV, confirming an effective charge transfer from fragment 2 to fragment 3.In case of B, fragment 2 is found to have the greatest HOMO density at 7.2 eV whereas fragment 1 has the highest LUMO density.For C, fragment 1 depicts the highest maximum HOMO density of about 7.1 eV, whereas fragment 3 exhibit the highest maximum LUMO density of around 0.5 eV.This type of charge density favours the charge transfer from HOMO of fragment 1 to LUMO of fragment 3 via fragment 2.These findings demonstrate the importance of manipulating various components in fragment 3 to create diverse chromophores, influencing the transmission of electronic charges through distinct pathways.Overall, the results of DOS analysis significantly correlate with the outcomes of the FMO study.

Transition density matrix (TDM) analysis
The TDM is widely used to illustrate hole-electron pair dynamics and to explore nature of transitions in the examined compounds 73 .TDM analysis assists in the evaluation of electronic charge excitation processes, transitions from the ground state (S 0 ) to excited state (S 1 ), and electron-hole pair localization and delocalization.The TDM heat maps provide information about the nature of transition in these synthesized chromophores 47 .Figure 5 displays the transition density matrix (TDM) maps for all synthesized compounds, depicting the charge density transfer between the three distinct components (fragments 1, 2, and 3) of each chromophore.It is worth noting that the influence of hydrogen atoms on electronic transitions is not discussed in this study due to their minimal contribution.
Figure 5 depicts that in compounds A and B, the higher electronic cloud is clearly visible by green and red dots on fragment 3 and 2 of the heat maps.This electronic cloud exploited the successful transfer of electrons.www.nature.com/scientificreports/However, compound C heat map shows that the electronic cloud is primarily on fragment 2, with a small amount of charge also visible on fragment 3.Such a pattern of electronic cloud facilitates the electron density to migrate towards fragment 3 without trapping.The presence of the very electron-deficient group (-NO 2 ) on fragment 3 of compound C may be the reason for this peculiar behavior 74 .The TDM heat maps show efficient charge transfer coherence without any trapping in any of the investigated chromophores.These visual representations of TDM indicate a more straightforward and enhanced exciton dissociation in the excited state, facilitating the advancement of materials for NLO applications.

Natural population analysis (NPA)
The computation of effective atomic charges, which shows the distribution of positive and negative charges among atoms in the molecule, is essential for determining whether the length of the bonds between the atoms should increase or decrease.Atomic charges have an impact on the dipole moment, electrostatic potential surface, electronic structure, molecular polarizability, and numerous other properties of molecular systems 75,76 .Figure 6 demonstrates the results of an NPA analysis to determine the natural charges of A-C compounds.The process of charge transformation that results from reactions, as well as electrostatic potential on system surfaces are all commonly assessed using the natural charge examination 77 .The arrangement of molecules and their bonding capabilities are heavily impacted by the electrical charges carried by individual atoms.Natural charges of the molecules under investigation reveals that the presence of electronegative elements such as O, F, Cl, and Br are responsible for an uneven redistribution of the electron density across the benzene rings 78 .Additionally, Mulliken population analysis confirm that all of the hydrogen atoms have the same charge distribution, with the negative charges on carbon atoms leading to positive charges on hydrogen atoms.
From NPA, it can be observed that positive charges are present on all H, S, and Br-atoms.However, some C atoms have a positive charge while others have a negative charge.The presence of large negative charge on C atoms is due to their involvement in resonance with neighbouring N and O atoms.All N, Cl, and F-atoms were negatively charged except one nitrogen in compound C.Here it can be seen that C34 of compound A, C28 of www.nature.com/scientificreports/compounds B and C showed a higher positive charge while N32 of compound A, N31 of compound B and C has a higher negative charge (see Fig. 6).

Natural bond orbital (NBO) analysis
NBO analysis confirm the presence of hydrogen bonds resulting from hyper-conjugated interactions in the investigated chromophores 79 .Because all the orbitals are mathematically selected to include the highest possible percentage of the electron density, it provides the most precise representation of the natural Lewis structure.
The NBO analysis effectively explains the interaction between the filled and virtual orbital space information 80 .In Eq. ( 8), the stabilization energy E (2) related to the delocalization i → j for each donor (i) and acceptor (j) is calculated by second-order perturbation theory 81 .
where F(i.j) is the off-diagonal NBO Fock matrix member, i and j are diagonal elements, and q i is the donor orbital occupancy, while, E i and E j illustrate off-diagonal NBO Fock or Kohn-Sham medium elements 82,83 .The stabilization energies of several orbitals have been examined and are presented in Tables S2-S4, while important transitions are depicted in Table 3.

Nonlinear optical (NLO) properties
The development of materials with improved NLO behavior is required for the development of electro-optic modulation, frequency mixing, enhanced data rates, and harmonic generation related to the technology of potential communication 85,86 .Champagne and Bishop developed an intriguing topic by examining the effects of NLO on organic molecules 87 .Organic polymeric as well as heterocyclic compounds with significant hyperpolarizability amplitudes have attracted attention due to their potential use in NLO materials.Many research efforts develop effective methods for synthesizing organic chromophores in the NLO field because of their distinctive properties 88 .Compounds A-C have all been examined in the present study for possible NLO features by computational and experimental calculation.Based on the electronic characteristics, it is estimated that the intensity of the optical reactions would record a progressive linear as well as nonlinear responses 89 .The dipole moment (µ tot ), polarizability < α > , first-order hyper-polarizability (β tot ), and second-order hyper-polarizability (γ tot ) values of the synthesized compounds computed by Eqs. ( 9)-( 12) are shown in Table 4. Tables S5-S8 demonstrate the detailed outcomes for all tensors.www.nature.com/scientificreports/An essential consideration for determining the polarizability of organic chromophores is the dipole moment (µ tot ) 90 which is the measure of the separation of charges.The µ tot of chromophores A-C is found to be 5.606, 4.222, and 5.866 D, respectively.Compound C has the highest total value of all the compounds (5.866 D), which may be due to the presence of a potent electron-withdrawing group NO 2 .On the other side, B which has a weak electron withdrawing chloro group in fragment 3, has the lowest value of dipole moment at 4.222 D. The overall increasing order of dipole moment is B < A < C. The prominent values of µ y (4.697, 3.718, and 5.066 D, respectively), shown in Table S5, suggest that the greater polarity for compounds A-C resides along the y-axis (ordinate).Furthermore, on comparing the outcomes with standard para-nitroaniline chromophore (µ para-nitroaniline = 6.3 D), it is observed that the newly synthesized compounds exhibit better polarity than standard p-NA.
The average polarizability was observed to be the highest in compound A (4.324 × 10 -23 esu).The lowest value of < α > , on the other hand, was found in B (4.109 × 10 -23 esu).This was because of the presence of the CF 3 group along with the chloro group that enriched the electron density and improved the electron-withdrawing capacity.The < α > values of A-C are 4.324 × 10 -23 , 4.109 × 10 -23 , and 4.195 × 10 -23 esu, accordingly and tensors of < α > are duslpayed in Table S6.According to the literature, molecule polarizability is influenced by the HOMO-LUMO energy gap 91 .The HOMO-LUMO energy gap is inversely related to both linear and nonlinear polarizabilities.Small HOMO-LUMO energy gap compounds enable high nonlinear and linear polarizabilities.In contrast to the energy gap values for A, B, and C, a small energy gap for C is seen in our analysis.As a result, its linear and nonlinear polarizability values are higher.
Among all of the developed compounds, the highest value of β tot was observed in C at 6.317 × 10 -30 esu as it has a powerful electron-withdrawing unit − NO 2 .A slight decrease in β tot value was found in A i.e., 5.565 × 10 -30 esu.However, the lowest value of β tot was observed in B (5.240 × 10 -30 esu).First-order hyper-polarizability responses of A-C were 401.5616, 1605.112, and 1298.838times greater than that of the p-NA (3.610 × 10 -34 ), respectively.
For each of the investigated chromophores, a notable secondary hyperpolarizability was observed in the y-axis direction, as illustrated in Table S7, with C having a greater value of γ tot among the derivatives.The decreasing order of second-order hype-polarizability (γ tot ) values is shown as: C > A > B. In a comparative investigation, we also studied the non-linear optical characteristics of p-NA (3.812 × 10 -36 ) used as the standard chromophore.This comparison with p-NA demonstrates that compounds A-C, with compound C being the most effective among them, are efficient candidates for NLO materials.The reasoning above also leads to the conclusion that introducing different groups in fragment 3 results in yielding significant NLO amplitude.

Conclusion
In the current research, synthesis of three novel N-substituted 2-((5-(3-bromophenyl)-4-methyl-4H-1,2,4-triazol-3-yl)thio)-N-phenylpropanamide derivatives have been reported in multiple step approach.The structures of these compounds were analyzed by spectroscopic analysis.These spectral analyses provided crucial evidence supporting the structural integrity of the synthesized compound.All synthesized hybrids (7a-7c) exhibited UV-Vis absorption spectrum within the range of 298 nm, 297 nm, and 295 nm.IR spectra confirmed the presence of specific functional groups peaks associated with NH groups 3272-3158 cm −1 , carbonyl groups (C=O) 1733-1677 cm −1 , N=C 1694-1641 cm −1 , and N=N 1596-1526 cm −1 , and aromatic C=C stretching 1480-1440 cm −1 while NMR spectra detected quartets, singlets, and doublets, at 4.58-4.82,3.55-3.65,and 1.71-1.75ppm, and H-4′ and H-5′ contained triplets at 7.39-7.67ppm in all compounds' spectra (7a-7c).According to the FMO data, compound C had the lowest band gap of 4.618 eV.Additionally, global reactivity parameters were also correlated to the band gap, compound C exhibits the lowest global hardness of 2.309 eV and the highest softness value of 0.216 eV.Furthermore, higher values of NBO-based hyper conjugative interactions were noted, endorsing the highest stability of the molecules under investigation.The charge distribution and electrophilic and nucleophilic areas of the examined chromophores were explained by natural population analysis According to TDM heat maps, the reduced band gap will help the effective transmission of charges from fragment 2 to fragment 3. DOS pictographs provide more support to this efficient transfer of electronic cloud.Moreover, the synthesised compound C exhibits a notable large NLO response with values of 5.866 D, 4.195, 6.317

Figure 3 .
Figure 3.The HOMOs and LUMOs of the compounds (A)-(C), illustrate the electronic cloud over the orbitals.

Figure 4 .
Figure 4.The DOS plots of the titled compounds (as segmented into three fragments: 1, 2, and 3 are shown with red, green, and blue lines, respectively).

Figure 6 .
Figure 6.Natural population analysis graph of all the synthesized compounds illustrated the presence of electronegative elements.

Table 1 .
The E HOMO , E LUMO, and E of A-C, units in eV.

Table 3 .
Chosen NBO analysis results for the examined chromophores (A-C).